The invention relates to a circuit arrangement for feeding a load, which is equipped with
input terminals which are to be connected to a supply voltage source,
a power converter for generating a current through the load from a supply voltage supplied by the supply voltage source,
a control circuit for controlling the root-mean-square (rms) value of an operating parameter, comprising
a first circuit for generating a first signal that is a measure of the actual rms value of the operating parameter,
a second circuit for generating a second signal that is a measure of the desired rms value of the operating parameter,
a third circuit that is coupled to the first and the second circuit for generating a third signal that is dependent on the first and the second signal, and for influencing the operating state of the circuit arrangement in dependence on the third signal.
Such a circuit arrangement is known. The first signal is frequently generated by successively rectifying and averaging the signal that represents the operating parameter. As regards sinusoidal signals, it applies that Vrms=1.111 * Vavg, where Vrms is the rms value of the signal and Vavg is the average value, so that said operation yields a reliable rms value for sinusoidal signals. However, if said operation is applied to a signal that deviates substantially from a sine-shaped signal, the result of the operation may also deviate substantially from the actual rms value of the signal. To determine the rms value of such a non-sinusoidal signal use can be made of a xe2x80x9ctrue rms sensorxe2x80x9d. Such a xe2x80x9ctrue rms sensorxe2x80x9d however is a complicated circuit comprising many (active) components, as a result of which said circuit is also comparatively expensive.
It is an object of the invention to provide a circuit arrangement wherein the rms value of an operating parameter can be determined by means of comparatively simple means and regulated so as to obtain a desired level.
To achieve this, a circuit arrangement as mentioned in the opening paragraph is characterized in accordance with the invention in that the first circuit is equipped with
a fourth circuit for generating a fourth signal that is a measure of the actual average value of the operating parameter,
a fifth circuit for generating a fifth signal that is a measure of the actual maximum amplitude of the operating parameter,
a sixth circuit for generating a signal that is a linear combination of the third signal and the fourth signal.
It has been found that the rms value generated by the first circuit of a circuit arrangement in accordance with the invention is substantially equal to the actual rms value of the operating parameter, even if the form of the operating parameter as a function of time deviates substantially from the sine shape. By virtue thereof, also the rms value of the operating parameter is accurately regulated so as to obtain the desired value. The fourth circuit and the fifth circuit can be obtained using comparatively simple electronics, so that these circuits are also comparatively inexpensive. The same applies to the sixth circuit.
Good results were achieved using embodiments of a circuit arrangement in accordance with the invention wherein the load is a lamp, and the operating parameter is selected among the group consisting of the lamp current, the lamp voltage and the power consumed by the lamp.
Good results were also achieved using embodiments of a circuit arrangement in accordance with the invention wherein the sixth circuit generates a signal of the general formula
A*OPavg+B*OPmax,
wherein OPavg and OPmax are the value of the fourth signal and the value of the fifth signal, respectively, and wherein
0 less than A less than 1
and
0 less than B less than 1.
More particularly, good results were achieved for embodiments wherein
0.76 less than A less than 0.93
and
0.12 less than B less than 0.19.
These and other aspects of the invention are apparent from and will be elucidated with reference to the embodiment(s) described hereinafter.